Location of a subscriber terminal, i.e. determination of the geographical location of a subscriber terminal, is an important function in cellular radio networks. In the United States a federal authority (Federal Communication Commission FCC) requests that one should be able to locate all subscriber terminals that make emergency calls even with accuracy of 50 meters. Location can also be used for commercial purposes, e.g. for defining various tariff areas or for implementing a navigation service which guides the user. So far the location service (LCS) has been mainly developed for circuit-switched cellular radio networks, such as the GSM system (Global System Mobile Communications).
Various methods are used for implementing the location service. At the least accurate level the location of a subscriber terminal can be determined on the basis of the identity of the cell that serves the subscriber terminal. This does not provide very accurate information because the diameter of a cell can be dozens of kilometers.
A more accurate result is obtained by using timing information on the radio connection as additional information, e.g. the timing advance TA. In the GSM system the TA provides the location of a subscriber terminal with accuracy of approximately 550 meters. The problem is that if the cell is implemented with an omnidirectional antenna, the location of a subscriber terminal with respect to a base station is known only on the circumference drawn around the base station. A base station divided into three sectors, for example, slightly improves the situation, but even in that case it can only be determined that the subscriber terminal is located in a 120° sector in an area with a depth of 550 meters at a certain distance from the base station.
These inaccurate methods are adequate for some applications, e.g. for determining tariff areas. More accurate methods have also been developed. These methods are usually based on various base stations measuring a signal transmitted by a subscriber terminal. One example of such methods is the TOA method (Time of Arrival).
The subscriber terminal can also measure signals transmitted by various-base stations. An example of such a method is the E-OTD method (Enhanced Observed Time Difference). In synchronized networks the subscriber terminal measures the ratios between the reception moments of the signals it has received from different base stations. In non-synchronized networks the signals transmitted by the base stations are also received by a location measurement unit LMU placed at a known and fixed measurement point. The location of the subscriber terminal is determined on the basis of geometrical components obtained from time delays.
Another location method is to use a GPS receiver (Global Positioning System) arranged in a subscriber terminal. The GPS receiver receives a signal transmitted by at least four earth-orbiting satellites, and the latitude, longitude and altitude of the location of the subscriber terminal can be calculated on the basis of these signals. The subscriber terminal either determines the location independently or it is assisted in the determination. The network part of the radio system may send an assistance message to the subscriber terminal, which speeds up the location process, i.e. the power consumption of the subscriber terminal decreases. The assistance message may contain the time, a list of visible satellites, Doppler shift of the satellite signal and a search window for the code phase. The subscriber terminal can send the information it has received to the network part, in which the location is calculated. In this application the network part of the radio system refers to the fixed part of the radio system, i.e. to the whole system except for the subscriber terminal.
In packet-switched radio systems, like in the GPRS (General Packet Radio Service) or in the EGPRS (Enhanced General Packet Radio Service), very little attention has been paid to the implementation of the location service. The EGPRS is a GSM-based (Global System for Mobile Communications) system which utilizes packet-switched transmission. The EGPRS employs the EDGE (Enhanced Data Rates for GSM Evolution) technique for increasing the data transmission capacity. In addition to the GMSK modulation (Gaussian Minimum-Shift Keying) normally used in the GSM, the 8-PSK modulation (8-Phase Shift Keying) can also be applied to packet data channels. The main object is to provide non-real-time data transmission services, such as copying of files and use of an Internet browser, but also packet-switched real time services for transmission of speech and video, for example.
In packet-switched radio systems the location methods described above require a packet-switched transmission connection between the core network of the radio system and the subscriber terminal for transmitting the necessary information. The signaling traffic needed is rather heavy and slow. In time critical applications it would, however, be important to receive the location of the subscriber terminal fast from the location service.